Mercury methylation and demethylation potentials in Arctic lake sediments

Abstract Mercury (Hg) transformations in sediments are key factors in the Hg exposure pathway for wildlife and humans, yet are poorly characterized in Arctic lakes. As the Arctic rapidly warming, it is important to understand how the rates of Hg methylation and demethylation change with temperature in sediments, which determine Hg bioavailability in Arctic lakes. Methylation and demethylation potentials were determined for littoral sediments (7 m water depth) in two deep and two shallow lakes in the Canadian Arctic using Hg stable isotope tracers at incubation temperatures of 4, 8, or 16 ˚C for 24 hrs. Compared to sediments from other regions, Hg methylation and demethylation potentials in these sediments are low. The maximum depth of the lake from which sediment was collected exerted a stronger influence over methylation potential than sediment Hg concentration or organic matter content; the shallowest lake had the highest Hg methylation potential. Sediments from the shallowest lake also demonstrated the greatest response to the temperature treatments, with significantly higher methylation potentials in the 8 and 16 ˚C treatments. Sediments from the deep lakes demonstrated greater demethylation potentials than shallow lakes. The methylmercury to total Hg ratio in sediments supported the measured transformation potentials: as the lake with the greatest methylation potential had the ratio. This study supports previous works indicating that Hg methylation potential may increase as the Arctic warms, but demethylation potential does not respond to warming to the same degree, indicating that Hg methylation may predominate in warming Arctic sediments.